Technical Field
The technical field of this disclosure relates generally to building control systems for heating, ventilation, and air conditioning systems, and more specifically, to systems and methods for obtaining an air flow characteristic that may be used by a control system.
Background Discussion
HVAC systems are often used to regulate environmental conditions, such as air temperature and humidity, in one or more control zones within a building or other structure. Such HVAC systems typically include a controller that controls various components of the HVAC system in order to affect the temperature or humidity of the air within the building. Controllers for HVAC systems may be configured to conserve energy by adjusting the setpoint temperature of a control zone during periods of non-occupancy. For instance, during the winter months, heating may be disabled during periods of non-occupancy and the space temperature is allowed to drop. Conversely, during the summer months, cooling may be disabled during periods of non-occupancy, and the space temperature is allowed to increase. When heating or cooling is disabled, the space temperature may therefore be allowed to reach a natural level as determined by outside conditions and/or any sources of energy located within the building.
In order for the control zone to achieve a desired temperature at the start of a period of occupancy (referred to herein as the target start of occupancy temperature ti) HVAC controllers may include an algorithm for determining at what time prior to occupancy one or more components of the HVAC system must be activated. This time period is often referred to as the preheat period (also referred to herein as the preheat time interval), although the term also applies in instances where the air is cooled, rather than heated. To illustrate, FIG. 1 is a graph showing temperature values in a controlled space (otherwise referred to herein as a control zone) during a period of non-occupancy. At time Θ1, the period of occupancy ends with the temperature in the controlled space being at the desired setpoint temperature td, and the period of non-occupancy begins. The temperature drops to tp until time Θ2, when the HVAC system is turned on to raise the temperature to ti by the time of occupancy re-commences at Θ3. The period between Θ2 and Θ3 is known as the preheat period, and is sometimes also referred to as the “boost” or “start-up” period. Time Θ2 therefore denotes the start of the preheat period and Θ3 denotes the end of the preheat period. According to the example shown in FIG. 1, the temperature may continue to rise from the start of occupancy temperature ti to the setpoint temperature td after the time of occupancy re-commences at Θ3. In certain instances, the target temperature at the start of occupancy ti may be lower than the target setpoint temperature td because occupants may accept a lower temperature than the target setpoint temperature at the beginning of the occupancy period as compared to later on in the occupancy period. For example, occupants entering the controlled space from cold outside temperatures may accept a lower indoor temperature initially out of thankfulness for being out of the cold. Similar logic also applies to cooling, where the target temperature at the start of occupancy ti may be higher than the setpoint temperature td.
Miscalculation of the preheat period can have negative consequences. For instance, if the preheat period is too short, the start of occupancy temperature is not achieved, and if the preheat period is too long, the start of occupancy temperature is achieved prematurely with unnecessary use of energy.